Ceramic honeycomb catalyst carrier and method of producing the same

ABSTRACT

A ceramic honeycomb catalyst carrier wherein a porosity is 20% or less and an average surface roughness (Ra) of a partition wall of the carrier is 0.5 μm or more is obtained by forming ceramic raw materials to obtain a ceramic honeycomb formed body; drying the ceramic honeycomb formed body to obtain a ceramic honeycomb dried-up body; roughening a surface of the partition wall of the ceramic honeycomb dried-up body by exposing the ceramic honeycomb dried-up body in an airflow in which polishing powders are included; and sintering the ceramic honeycomb dried-up body after a surface treatment, or, by forming ceramic raw materials to obtain a ceramic honeycomb formed body; drying and sintering the ceramic honeycomb formed body to obtain a ceramic honeycomb sintered body; and roughening a surface of the partition wall of the ceramic honeycomb sintered body by exposing the ceramic honeycomb sintered body in an airflow or a water flow in which polishing powders are included.

TECHNICAL FIELD

The present invention relates to a ceramic honeycomb catalyst carrierused for purifying an exhaust gas from automobile and so on and a methodof producing the same.

BACKGROUND ART

A ceramic honeycomb catalyst carrier is used for fixing a catalyst forpurifying an exhaust gas from automobile and so on. Generally, theceramic honeycomb catalyst carrier has a plurality of partition wallseach having a number of pores, on which γ-alumina having a high specificsurface area and a catalyst are coated, thereby being used forautomobile and so on. In view of a recent beef-up of exhaust gasregulation, it is required to make a thickness of the partition wall ofthe ceramic honeycomb catalyst carrier thin so as to improve a catalystperformance and an engine performance.

However, if the partition wall of the ceramic honeycomb catalyst carrieris made thin as mentioned above, a mechanical strength of the ceramichoneycomb catalyst carrier is decreased. In order to eliminate such aproblem, in Japanese Patent Laid-Open Publication No. 10-174885(JP-A-10-174885) for example, a technique, wherein a dense cordieritehaving a high strength and a porosity of 18% or less is obtained bycontrolling a raw material size of talc and an amount of CaO, isdisclosed. However, if a porosity is decreased, a strength can beimproved, but there arises a new problem such that a coatability ofγ-alumina having a high specific surface area and catalyst isdeteriorated.

DISCLOSURE OF INVENTION

An object of the invention is to eliminate the drawbacks mentioned aboveand to provide a ceramic honeycomb catalyst carrier having a sufficientstrength as a catalyst carrier and excellent catalyst coatability evenif a partition wall is thin and a method of producing the same.

According to the invention, a ceramic honeycomb catalyst carrier ischaracterized in that a porosity is 20% or less and an average surfaceroughness (Ra) of a partition wall of the carrier is 0.51 μm or more.

Moreover, according to a first aspect of the invention regarding aproducing method, a method of producing the ceramic honeycomb catalystcarrier mentioned above, comprises the steps of: forming ceramic rawmaterials to obtain a ceramic honeycomb formed body; drying the ceramichoneycomb formed body to obtain a ceramic honeycomb dried-up body;roughening a surface of the partition wall of the ceramic honeycombdried-up body by exposing the ceramic honeycomb dried-up body in anairflow in which polishing powders are included; and sintering theceramic honeycomb dried-up body after a surface treatment.

Further, according to a second aspect of the invention regarding aproducing method, a method of producing the ceramic honeycomb catalystcarrier mentioned above, comprises the steps of: forming ceramic rawmaterials to obtain a ceramic honeycomb formed body; drying andsintering the ceramic honeycomb formed body to obtain a ceramichoneycomb sintered body; and roughening a surface of the partition wallof the ceramic honeycomb sintered body by exposing the ceramic honeycombsintered body in an airflow or a water flow in which polishing powdersare included.

In the present invention, a ceramic honeycomb catalyst carrier, in whicha porosity is 20% or less and a partition wall of the carrier isroughened so as to be an average surface roughness (Ra) of 0.5 1 μm ormore, can be obtained by performing a surface treatment with respect toa surface of the partition wall preferably by means of polishing powderssuch as alumina and diamond after forming and drying ceramic rawmaterials or further after sintering a formed body. In this manner, itwas found that it is possible to obtain a ceramic honeycomb catalystcarrier having a high strength and excellent γ-alumina or catalystcoatability.

In the present invention, the reason for defining an average surfaceroughness as 0.5 1 μm or more is that, if it is less than 0.5μm,γ-alumina and catalyst are not sufficiently coated thereon. Moreover,the reason for defining porosity as 20% or less is that, if it is largerthan 20%, a sufficient strength can not be obtained. Further, thesurface treatment mentioned above may be performed by circulating aliquid, in which polishing powders such as alumina and diamond aredispersed, by means of a pump, or, by exposing a ceramic honeycomb for adefinite period of time in an atmosphere in which polishing powders areincluded in an airflow. Furthermore, as a material of the ceramichoneycomb catalyst carrier, use may be made of a popular ceramics suchas partially stabilized zirconia, silicon nitride, alumina, siliconcarbide, and zirconium phosphate, other than cordierite.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a flowchart for explaining a method of producing a ceramichoneycomb catalyst carrier according to a first aspect of the invention;and

FIG. 2 is a flowchart for explaining a method of producing a ceramichoneycomb catalyst carrier according to a second aspect of theinvention.

BEST MODE FOR CARRYING OUT THE INVENTION

FIG. 1 is a flowchart for explaining a method of producing a ceramichoneycomb catalyst carrier according to a first aspect of the invention.Hereinafter, explanations are made with reference to FIG. 1. At first,ceramic raw materials are formed so as to obtain a ceramic honeycombformed body, and the ceramic honeycomb formed body is dried so as toobtain a ceramic honeycomb dried-up body. Then, the ceramic honeycombdried-up body is subjected to a surface treatment. The surface treatmentaccording to a producing method of the second aspect of the invention isperformed by exposing the ceramic honeycomb dried-up body for apredetermined period of time in an airflow in which polishing powderssuch as alumina, diamond and so on are included. Then, the ceramichoneycomb dried-up body after the surface treatment is sintered. In thismanner, it is possible to obtain a ceramic honeycomb catalyst carrieraccording to the invention in which porosity is 20% or less and anaverage surface roughness (Ra) of a partition wall of the carrier is 0.5μm or more.

FIG. 2 is a flowchart for explaining a method of producing a ceramichoneycomb catalyst carrier according to a second aspect of theinvention. Hereinafter, explanations are made with reference to FIG. 2.At first, ceramic raw materials are formed so as to obtain a ceramichoneycomb formed body, and the ceramic honeycomb formed body is driedand sintered so as to obtain a ceramic honeycomb sintered body. Then,the ceramic honeycomb sintered body is subjected to a surface treatment.The surface treatment according to a producing method of the firstaspect of the invention is performed by circulating a liquid, in whichpolishing powders such as alumina and diamond are dispersed, by means ofa pump, or, by exposing a ceramic honeycomb for a definite period oftime in an atmosphere in which polishing powders are included in anairflow. In this manner, it is possible to obtain a ceramic honeycombcatalyst carrier according to the invention in which porosity is 20% orless and an average surface roughness (Ra) of a partition wall of thecarrier is 0.51 μm or more.

It should be noted that use is made of all the known methods as a methodfor making porosity of the carrier to 20% or less. For example, it ispossible to use a method for defining a raw material size of talc and anamount of CaO as disclosed in Japanese Patent Laid-Open Publication No.10-174885 (JP-A-10-174885). Moreover, in the thus obtained ceramichoneycomb catalyst carrier according to the invention, it is preferredthat a thickness of the partition wall is 60 μm or less and an openingrate of the honeycomb body is 88% or more. Further, it is preferred thatthe ceramic honeycomb catalyst carrier has ISO strength of 500 KPa ormore. Furthermore, if an average surface roughness of the partition wallsurface is 1.0 μm or more, it is possible to obtain further excellentcoatability. Therefore, it is also a preferred embodiment.

Hereinafter, actual experiments will be explained.

According to the following methods, a ceramic honeycomb catalyst carrierwas obtained by performing a producing of a honeycomb carrier and asurface treatment of the honeycomb carrier. After that, with respect tothe thus obtained ceramic honeycomb catalyst carrier, porosity, flexuralstrength, hydrostatic pressure fracture strength, surface roughness, andcoating amount were measured. Results of examples according to theinvention and comparative examples without a scope of the invention areshown in the following Table 1.

Producing of Honeycomb Carrier

Raw materials such as talc, alumina, kaolin were blended to obtain amixture having a cordierite composition. Then, with respect to themixture, a suitable amount of crushed cordierite as a micro porecontrolling agent, water-soluble cellulose derivatives, surface activeagent and water were added so as to obtain a batch. Then, according tothe known producing method, the batch was subjected to kneading,pugging, extrusion molding, drying and sintering so as to obtain aceramic honeycomb catalyst carrier having a cordierite composition and athickness of the partition wall shown in the following Table 1.

Surface Treatment of Honeycomb Carrier

A water-resistant fan was arranged in a tube which can perform anopening and closing motion, and the ceramic honeycomb sintered body wasset in the tube in such a manner that through holes defined by thepartition walls were aligned in a water flow direction of the fan. Then,a water flow was passed through the ceramic honeycomb sintered body bymeans of the fan. In this case, diamond polishing powders were dispersedbetween the fan and the ceramic honeycomb sintered body, thereby etchinga surface of the partition wall of the ceramic honeycomb sintered body.After the etching process, the ceramic honeycomb sintered body waswashed sufficiently and dried up so as to obtain a ceramic honeycombcatalyst carrier. The diamond polishing powders were collected so as torecycle them.

Porosity

According to a mercury injection method, a total micro pore capacity ofthe thus obtained ceramic honeycomb catalyst carrier was measured so asto calculate porosity. A true density of cordierite was assumed to be2.52 g/cm³. The measurement was performed by AutoPore 9405 bymicromeritics.

Flexural Strength

According to the same producing method as that of the honeycomb carriermentioned above, a cordierite flat plate having width of 30 mm,thickness of 7 mm and length of 150 mm was obtained. In this case, thesurface treatment with respect to the flat plate was performed aftersintering and machining processes. After finishing all the processes, afour point flexural strength was measured with respect to five sampleson the basis of JIS R1601.

Hydrostatic Pressure Fracture Strength

The thus obtained ceramic honeycomb catalyst carrier was inserted in aflexible tube, and a hydrostatic pressure due to a water pressure wasapplied thereto. Then, a pressure (KPa), at which a partial fractureoccurred, was measured. The number of samples was ten respectively.

Surface Roughness

An average surface roughness (Ra) of the partition wall of the thusobtained ceramic honeycomb catalyst carrier was measured by FTS-S4Cmanufactured by Taylor-Hobson Company. A calculation method is referredto JIS B0601. At a tip measurement portion, use was made of a 2 μmconical diamond.

Coating Amount

A coating amount (g/honeycomb cc) showing coatability of the thusobtained ceramic honeycomb catalyst carrier was measured as follows. Asample having a size of 100 mm×100 mm×100 mm was cut out from theceramic honeycomb catalyst carrier. At the same time, 5% ofalumina-nitrate sol and γ-alumina were mixed in a rate of 60:40 so as toprepare an alumina slurry. Then, the sample was immersed for apredetermined period of time in the alumina slurry. After that, thesample coated with the alumina slurry was dried and aged at 600° C. forone hour. The coating amount (g/honeycomb cc) showing coatability wascalculated on the basis of a weight increase before and after thealumina slurry coating.

TABLE 1 Example according to Comparative the invention example 1 2 3 4 12 Surface treatment Yes Yes Yes Yes No No Partition wall thickness of 5060 50 50 50 50 carrier (μm) Porosity of carrier (%) 18 18 18 20 32 20Surface roughness (μm) 2.0 1.5 0.5 0.7 1.9 0.3 Flexural strength (MPa)42 42 43 38 21 40 Hydrostatic pressure fracture 1370 1450 1380 1180 5901200 strength (KPa) Coating amount (g/honeycomb cc) 0.20 0.17 0.10 0.130.25 0.05

From the results shown in Table 1, the followings were understood. Atfirst, in the example 1 according to the invention, when the surfacetreatment was performed and the surface roughness is 2.0 μm, it ispossible to obtain a high coating amount and a low porosity, so that ahigh strength can be obtained. When the surface roughness was decreased,the coating amount was lowered in such a manner that the coating amountwas 0.1 g/honeycomb cc at the surface roughness of 0.5 μm (the example 3according to the invention). When the coating amount was decreased to alevel of 0.05 g/honeycomb cc as shown in the comparative example 2, itwas necessary to repeat the coating process again and again so as toobtain a sufficient coating amount necessary for a production, and thusit is not preferred. As seen from the examples 2 and 4 according to theinvention, a decrease of partition wall thickness and an increase ofporosity lead to a decrease of strength. When the porosity was very highas shown in the comparative example 1, the surface roughness wasincreased and at the same time the coating amount became very high.However, the hydrostatic pressure fracture strength was low.

From the results mentioned above, it was found that sufficient flexuralstrength, sufficient hydrostatic pressure fracture strength andsufficient coating amount could be obtained in the examples 1-4according to the invention in which the porosity of the carrier was 20%or less and the surface roughness was 0.5 μm or more, and that strengthsor coating amount were not sufficient in the comparative examples 1 or 2in which either conditions of the porosity and the surface roughness wasnot satisfied.

In the examples 1-4 according to the invention, the surface treatmentwas performed for the ceramic honeycomb sintered body to obtain theceramic honeycomb catalyst carrier, but it is possible to obtain thesame results for the ceramic honeycomb catalyst carrier produced in sucha manner that the surface treatment was performed for the ceramichoneycomb dried body.

Industrial Applicability

As clearly understood from the above explanations, according to theinvention, a ceramic honeycomb catalyst carrier, in which a porosity is20% or less and a partition wall of the carrier is roughened so as to bean average surface roughness (Ra) of 0.5 μm or more, can be obtained byperforming a surface treatment with respect to a surface of thepartition wall preferably by means of polishing powders such as aluminaand diamond after forming and drying ceramic raw materials or furtherafter sintering a formed body. Therefore, it is possible to obtain aceramic honeycomb catalyst carrier having a high strength and excellentγ-alumina or catalyst coatability.

What is claimed is:
 1. A ceramic honeycomb catalyst carriercharacterized in that a porosity is 20% or less and an average surfaceroughness (Ra) of a partition wall of the carrier is 0.5 μm or more. 2.The ceramic honeycomb catalyst carrier according to claim 1, wherein athickness of the partition wall is 60 μm or less.
 3. The ceramichoneycomb catalyst carrier according to claim 1, wherein the carrierincludes a cordierite composition as a main ingredient.
 4. The ceramichoneycomb catalyst carrier according to claim 1, wherein an averagesurface roughness (Ra) of the partition wall of the carrier is 1.0 μm ormore.
 5. A method of producing the ceramic honeycomb catalyst carrierset forth in claim 1, comprising the steps of: forming ceramic rawmaterials to obtain a ceramic honeycomb formed body; drying the ceramichoneycomb formed body to obtain a ceramic honeycomb dried-up body;roughening a surface of the partition wall of the ceramic honeycombdried-up body by exposing the ceramic honeycomb dried-up body in anairflow in which polishing powders are included; and sintering theceramic honeycomb dried-up body after a surface treatment.
 6. A methodof producing the ceramic honeycomb catalyst carrier set forth in claim1, comprising the steps of: forming ceramic raw materials to obtain aceramic honeycomb formed body; drying and sintering the ceramichoneycomb formed body to obtain a ceramic honeycomb sintered body; androughening a surface of the partition wall of the ceramic honeycombsintered body by exposing the ceramic honeycomb sintered body in anairflow or a water flow in which polishing powders are included.